Charge image storage method and apparatus

ABSTRACT

The information stored on the dielectric target of a bistable storage tube is read out nondestructively employing a pulsed electron beam. The electron beam is turned on for a period having a duration less than necessary to change a given area of the target from one stable stored potential to another, while the period between pulsations is long enough so the tube&#39;s flood beam can return a given area to its original stored potential..Iadd.

This is a continuation of application Ser. No. 106,613 filed Jan. 14,1971, now abandoned which is a reissue of U.S. Pat. No. 3,426,238..Iaddend.

This invention relates to a charge image storage method and apparatusand particularly to such a method and apparatus providing high qualitynon-destructive read-out signals.

Information may be stored on the dielectric target of a cathode ray tubetype device in the form of a plurality of electric charges. Thisinformation may be either digital, or pictorial as, for example,representing a curve or other visual information. Such a storage tubeusually includes a high velocity electron gun for writing information onthe dielectric target and one or more flood guns for retaininginformation on the target as a result of the secondary emissionproperties of the system. Information is then conventionally read out bymeans of a high velocity beam, either from the same gun or a gun similarto the one employed in writing the information. During read-out, chargeis placed on a particular target area and causes an output signal on asignal plate associated with the target in accordance with theinformation previously stored. This method may destroy the informationread out since information is read out in the same manner it is written.

A high velocity electron beam employed for reading information from astorage target may be deflected to scan over the surface of the storagetarget in a raster fashion in order to sequentially pass over thevarious storage locations and read out any or all of the storedinformation. If the beam scans rapidly and infrequently over areas ofthe target bearing charge information, destruction of the informationstored will be avoided. However, when information is read out quiterapidly by means of a rapidly scanning electron beam, the informationmay encounter bandwidth problems when presented to ordinarycommunication circuitry for transmitting stored information from onelocation to another. The magnitude of the read-out current derived isalso dependent upon the scanning rate. As the scanning rate and the beamcurrent are decreased, the read-out current indicative of a particularelement or area of target information is decreased. The read-out signalcurrents are then small and difficult to amplify, taxing the state ofthe art amplifier technology.

It is therefore an object of the present invention to provide animproved charge image storage method and apparatus wherein storedinformation is nondestructively read out.

Another object of the present invention is to provide an improved chargeimage storage method and apparatus wherein information may benondestructively read out at a slow rate in order to accommodate lowbandwidth transmission systems and wherein the scanning beam may bestopped during its scan without destroying the information stored atthat location.

It is another object of the present invention to provide an improvedcharge image storage apparatus employing an electron beam for slowlyscanning the stored information.

It is another object of the present invention to provide an improvedcharge image storage method and apparatus wherein beam scanning of thestored information is not necessary.

It is another object of the present invention to provide an improvedcharge image storage method and apparatus for nondestructively readingstored information in a manner providing higher amplitude read-outsignals suitable for application to ordinary A.C. coupled amplificationapparatus.

In accordance with the method and apparatus of the present invention,the information stored on a .[.dielectric.]. storage tube target is readout employing a .[.relatively high velocity.]. pulsating electron beamfor interrogation. The interrogating electron beam may be eitherstationary or it may be scanned as desired in order to deriveinformation from individual locations or the entire target. The electronbeam is pulsed for periods having a duration less than .[.necessary tochange a selected area of the storage tube target from one stable storedpotential to another, that is, less than would be required to erase andwrite new information on the target. The.]. .Iadd.will cause erasure ofstored information. In a particular embodiment, the .Iaddend.periodbetween pulsations is .[.then.]. arranged to be sufficient for .[.the.]..Iadd.a .Iaddend.flood beam associated with the storage tube to drivethe area read back to its stable potential before another beam pulsationis received.

The subject matter which I regard as my invention is particularlypointed out and distinctly claimed in the concluding portion of thisspecification. The invention, however, both as to organization andmethod of operation, together with further advantages and objectsthereof, may best be understood by reference to the followingdescription taken in connection with the accompanying drawings whereinlike reference characters refer to like elements and in which:

FIG. 1 is a schematic diagram of a charge image storage apparatus inaccordance with the present invention,

FIG. 2 is a schematic diagram of charge image storage apparatus at alocation remote from that illustrated in FIG. 1 which is connected tothe FIG. 1 apparatus,

FIG. 3 is a plot of target potential versus signal plate potential in acharge image storage apparatus according to the present invention, and

FIG. 4 is a plot of target secondary emission ratio versus targetpotential for a charge image storage apparatus in accordance with thepresent invention.

Referring to FIG. 1, a charge image storage apparatus according to thepresent invention includes a storage tube envelope 10 formed ofinsulating material housing a principal electron gun including afilament 12, a cathode 14 connected to a high negative voltage source, acontrol grid 16, and a focusing and accelerating structure 18. Theelectron beam 20 produced by the principal electron gun is deflectedhorizontally by means of horizontal deflection plates 22 and verticallyby means of vertical deflection plates 24. The beam is directed towardsa target 46 at the opposite end of the tube. The storage tube isadditionally provided with one or more flood type electron guns 26 eachhaving a cathode 28, a control grid 30, and an anode 32, and which aresupported inside the envelope 10 adjacent the end of the verticaldeflection plates 24 closest the target. Cathodes 28 are convenientlymaintained at the zero volt level while grids 30 are suitably connectedto a -25 volts. Electrons emitted from the flood guns diverge into awide beam which is substantially uniformly distributed towards thetarget 46.

A plurality of electrodes are also provided on the inner surface ofenvelope 10 beyond the flood guns. A first electrode 34, connected tomidpoint of a voltage divider comprising resistors 36 and 38 coupledbetween a plus 250 volts and ground, acts to provide a more uniformelectric field to collimate electrons. A collector electrode 40 near thetarget end of the tube is connected at a midpoint of a voltage dividerincluding resistors 42 and 44 coupled between a plus 500 volts andground. This electrode can perform the additional function of collectingsecondary electrons as will hereinafter become more evident.

Storage target 46 is disposed on the inner side of glass end plate 48and includes a transparent signal plate 50 over which is disposed a.[.photosensitive.]. dielectric 52.[., suitably.]. .Iadd.which may bephotosensitive, for example .Iaddend.an integral layer of P-1 typephosphor. Signal plate 50 is a thin transparent conductive coating suchas tin oxide or the like and is connected to the midpoint of a voltagedivider comprising resistors 56 and 58 disposed between a plus 500 voltsand ground. The tube voltages are selected to result in the desiredsecondary emission characteristic of the tube as illustrated in FIG. 4where secondary emission ratio is plotted against target potential.

An amplifier 54, which may be of the alternating current coupled type,has its input coupled to signal plate 50. Amplifier 54 drives a Schmitttrigger circuit 57, the output of which is coupled to multivibrator 59.The Schmitt trigger circuit 57 and multivibrator 59 operate as a shapingcircuit or pulse stretcher to provide an output pulse 60 of uniformlength and magnitude at a time when the input voltage threshold ofSchmitt trigger circuit 57 is exceeded. Output pulse 60 of multivibrator59 is applied to mixer 61 in common with an output of scan andsynchronization generator 62.

The scan and synchronization generator 62 principally provideshorizontal and vertical scanning signals to horizontal and verticalplates 22 and 24 respectively of the storage tube. These scanningsignals are suitably of the same general type employed in televisionpractice and cause electron beam 20 to systematically and periodicallyscan target 46 in a scanning raster. The synchronizing signalscorresponding to such scanning raster are applied to mixer 61 by way oflead 64. The synchronization signals 66 are indicated at the output ofmixer 61, mixed with the output of multivibrator 59. The output of mixer61 is coupled to bandpass filter 68 and from there to a telephone line70 employed for communication with a remote location.

Control grid 16 of the principal electron gun is connected via doublethrow switch 72 alternatively to write generator 74 or an interrogatepulse generator 76. Write generator 74 applies the signal voltage togrid 16 for writing information charges by means of beam 20 on storagetarget 46 through the process of secondary emission. Write generator 74provides its output at the same time beam 20 is directed towards orscans over a selected area or target element where information is to bewritten. With switch 72 thrown in the opposite position, interrogatepulse generator 76 provides a series of short duration negative voltagepulses 78 employed for reading information whereby an output is providedon signal plate 50. The pulses are such that information is neitherwritten nor destroyed on the storage target 46, as hereinafterdescribed. The repetition frequency of pulses 78 is adjusted to be high,for example, an order of magnitude higher than the repetition rate ofinformation over which electron beam 20 scans during reading. This ratecannot, however, exceed the maximum bit rate of the output system unlessthis signal is later transformed to fall within the system's limits.

During operation, the tube polarities are arranged such that beam 20 hasa relatively high velocity for writing and is capable of producingsecondary electrons when it strikes storage dielectric 52. Secondaryelectrons are then suitably collected by collector 40 in which case thepotential of collector electrode 40 is suitably adjusted to be justslightly higher than the potential of signal plate 50. The storagedielectric 52 may alternatively have a sufficiently porous structure toenable secondary electrons emitted from the bombarded surface ofdielectric 52 to be transmitted therethrough and be collected.

The production of secondary electrons from an elemental area ofdielectric 52 on target 46 causes such area to become relativelypositive during operation of the present invention. Such area isretained at a relatively positive potential after beam 20 is scannedpast such elemental area because of the action of flood guns 26. Floodguns 26 produce relatively low velocity electrons which strike thetarget but which ordinarily have insufficient velocity for writinginformation thereon. When the electrons from flood guns 26 strike areasof the target upon which a positive charge has not been written, theseflood electrons tend to maintain such areas at the relatively negativepotential of the flood gun, e.g., zero volts. However, the flood gunelectrons are attracted by positive elemental areas and obtain a highvelocity with respect to these areas producing continued secondaryemission therefrom such that these areas are maintained relativelypositive or near the potential of signal plate 50 and collectorelectrode 40. The target thus has bistable properties and is capable ofretaining information written thereon, with the flood beam of electronsdriving target areas toward one of two stable potentials depending uponthe information written thereon with beam 20.

.[.Since.]. .Iadd.If .Iaddend.the phosphor storage dielectric 52 isphotosensitive, a light image directed onto the storage dielectric,through end plate 48 and transparent signal plate 50, produces a chargeimage on the storage dielectric corresponding to such light image. Thelight image produces charge carriers in the dielectric such thatilluminated areas of the target dielectric become more conductive andtend towards the more positive potential of signal plate 50. A lightimage of sufficient intensity directed towards target 46 will bemaintained through the action of the flood guns in the same manner as animage written on the target by means of electron beam 20. In order toenhance the photosensitive characteristics of the phosphor storagedielectric, photoconductive material such as cadmium sulphide or zincoxide may be uniformly mixed throughout the phosphor layer.

As previously mentioned interrogate pulse generator 76 causes apulsation of electron beam 20 wherein the pulses are of a predeterminedduration and rate such that information is read out from signal plate 50nondestructively, i.e., such that information is not read on to thetarget nor taken therefrom. During operation, when the electron beam 20strikes an area of the target where information is written in the formof a positive charge, an output pulse as indicated at 53 for examplewill be produced corresponding to each such pulse 78. The pulse is ofrelatively short duration and for this reason is applied to the pulseshaping or stretching circuitry including Schmitt trigger circuit 57 andmultivibrator 59. Schmitt trigger circuit 57 produces an output whenpulse 55 exceeds a predetermined voltage level and continues to producesuch an output until pulse 55 drops below a second predetermined voltagelevel. Multivibrator 59, which is suitably a one-shot multivibrator,provides pulse 60 of uniform amplitude and duration for each such inputpulse 55 applied to the Schmitt trigger circuit 57. The output pulse 60of multivibrator 59 is long enough so that it can be received by andcommunicated through ordinary communication equipment. Thus, a pluralityof output pulses 60, as mixed with synchronization signals 66, issuitably applied to a telephone line 70 through bandpass filter 68.Bandpass filter 68 suitably excludes 60-cycle and low frequency humcomponents. As will hereinafter become more evident, the scanning rateof electron beam 20 in the storage tube may be as slow as describedwithout resulting in problems of destructive read-out. Since slow scanrates are possible, the video information may be transmitted throughcommunications equipment of nearly any bandwidth.

The operation of the invention will be further described with referenceto FIGS. 3 and 4. A bistable storage device such as the present storagetube is in general characterized by hysteresis so that for one value ofan independent variable, there are two values of a dependent variable.Such a hysteresis curve for a storage tube is illustrated in FIG. 3where target dielectric potential is plotted against signal platepotential. The signal plate potential is the potential between the floodgun cathode 28 and signal plate 50. The potential between flood guncathode 28 and the scanned surface of dielectric 52 is the dependentvariable. .[.it.]. .Iadd.It .Iaddend.is noted the potential between theprincipal electron gun cathode 14 and signal plate 50 is always highenough to insure a secondary emission ratio greater than one forelectrons from the principal electron gun. Now if the signal platepotential is first raised to V_(c), in FIG. 3, the target potential issingle valued. If the signal plate potential is then lowered smoothly tothe value V_(a), the target potential is also single valued. However,moving the signal plate potential to V_(b), therebetween, allows thetarget potential to be double valued, but since the target potential wasat a uniform potential when the signal plate was at V_(a), it is nowstill at a uniform potential at the lower value indicated at 80. If we"write" a pattern on the target by preferentially raising selected areasto a higher potential level, e.g., by means of the principle of electrongun, we then store information and raise such areas to a potentialindicated at 82, because of secondary emission produced by beam 20.Writing can also be accomplished, of course, by lowering the resistivityof a target area by photoconductivity. Now that information has beenwritten on the target, it is maintained due to the action of the floodgun in retaining the negative polarity of unwritten areas and continuingsecondary emission from positive areas.

Examining the secondary emission ratio versus target potential curve forthe target acted on by the flood gun, which curve is illustrated in FIG.4, we see three points at which the secondary emission ratio is equal toone. At V_(d), δ=1, because the target, and specifically the insidesurface of dielectric 52, has collected sufficient electrons to charge afew tenths of a volt negative with respect to the flood gun cathode,thereby rejecting all electrons. At V_(e), the accelerating potential ishigh enough for the material on the target dielectric surface to emitsecondary electrons, and at V_(f) the target dielectric surface hascharged a few volts higher than the collector and all secondaryelectrons in excess of primary electrons are returned to the target.V_(d) and V_(f) are stable potentials. If the target begins to riseabove V_(d), the target collects electrons, the secondary emission beingless than one, and the target dielectric charges negatively restoringthe target dielectric to V_(d). If we bombard the target with a highenergy electron beam 20, and allow it to charge by secondary emission toany potential just under V_(e), it will return under the action of theflood guns to V_(d). However, if we allow it to charge more positivelythan V_(e), due to the action of beam 20, the secondary emission causedby the flood electrons will charge the target dielectric positivelyuntil it reaches V_(f). If it passes V_(f), the secondary emission ratiobecomes less than one and any electrons arriving tend to charge thetarget negatively. V_(e) is described as the first crossover voltage ofthe secondary emission characteristic.

Now, if we wish to read out or interrogate information stored on thetarget, we may do so by means of the same electron beam 20 which hasbeen employed to write information on the target, or, alternatively, wemay use a separate but similar electron beam. As when writinginformation with such electron beam, secondary emission is produced atthe target and information in the form of positive charge tends to bewritten. Thus information defined by the absence of a positive chargewould tend to be destroyed.

In accordance with the present invention, however, short reading pulses78 are applied to the grid 16 of the principal electron gun causingshort duration pulsation of the electron beam 20. The length of eachpulsation is selected such that the area to which the beam is directedis not entirely changed from one potential to another. The negative areais not rendered positive because the pulsation is shorter than requiredto change the selected area from V_(d) to V_(e) as illustrated in theFIG. 4 curve. That is, the pulsation of the electron beam 20 is shortenough so that the potential of the area being read out does not exceedthe first crossover point, V_(e) of the secondary emissioncharacteristic of the flood beam. Therefore, the flood beam drives sucharea back towards its original stable potential in order to retain thestored information. Mathematically t is less than: ##EQU1## In thisexpression, i_(p) =the high energy beam current, that is, the current ofbeam 20 during read-out;

i_(t) =the low energy flood beam current;

δ₁ =secondary emission ratio for i_(p) (greater than 1);

δ₂ =secondary emission for i_(t) (less than 1);

V_(e) =first secondary emission crossover;

C=target element capacitance at the area being read out;

t=time the high energy beam rests on the target element or area.

The time between pulsations of electron beam 20 is selected to be longenough for the target area read out to return to its original stablestate, for example, from V_(e) to V_(d). In general this time, T, shouldbe more than: ##EQU2## This is the time required for the target elementto return from V_(e) to approximately zero (V_(d) being only slightlyless than zero). However, if the pulsation time was materially less thanrequired to take the target to the crossover point V_(e), then theactual voltage to which the target area being read out was changedshould be substituted for a V_(e) in the above expression.

In the above expressions it will be noted that δ₁, and δ₂ vary duringthe times t and T, so the inequalities (1) and (2) are usually satisfiedempirically. In the case of an exemplary target employed, if a charge isdelivered to the target area which is less than approximately 10pico-couloumbs, it will be less than will write on such target. Thisvalue may be different for different targets.

The read-out obtained according to the present invention is in pulseswhich are uniform in amplitude, and width. The amplitude is dependentupon the current of beam 20, which is fixed, and the potential of thetarget. If we pulse electron beam 20 with a train of pulses with anon-time less than expression (1) and off-time greater than expression(2), this allows a stationary beam to interrogate the target areawithout writing or destroying information.

According to the present invention, a scan may also be applied tohorizontal deflection plates 22 and vertical deflection plates 24 sothat the entire target area is suitably read out. When the beam ismoving, the off-time may be shortened and the on-time may be lengthened,but the foregoing inequalities (1) and (2) must still be satisfied.Since we may interrogate with a stationary beam, it is seen that thescan rate is of virtually no consequence. Therefore, extremely slowscanning may be employed, for example, when it is desired to transmitinformation over a transmission system having low bandwidthcapabilities. The output signal is a pulse signal, and therefore, A.C.coupled amplifiers may be used throughout the system. Since the pulseoutput also has a characteristic period, amplifiers of the bandpass typemay be used as well as bandpass filter 68 so that interfering signalssuch as 60-cycle hum can be conveniently excluded. The read-out signalsare also of reasonable amplitude and may be conveniently amplified withordinary amplification circuitry.

FIG. 2 illustrates a circuit for storing information at a locationremote from the FIG. 1 charge image storage apparatus. Referring to FIG.2, telephone line 70, communicating with the FIG. 1 circuit, provides aninput for amplifier 84 which drives the control grid or other controlelectrode of a storage tube 86. Storage tube 86 is conveniently the sametype of storage tube as employed in the FIG. 1 apparatus. The output ofamplifier 84 is also applied to a sync separator 83 for derivingsynchronization signals 66 (see FIG. 1) from the input signal. Thesesignals synchronize scan generator 90 coupled to drive the deflectionapparatus of storage tube 86 in the same scanning pattern or rasterexecuted by the FIG. 1 apparatus.

In order to start operation of the communications system including thecircuits of FIGS. 1 and 2, a start signal is initiated by means of startsignal generator 92, transmitting a start pulse to scan andsynchronization generator 62 in FIG. 1 via telephone line 94. The scanand synchronization generator 62 causes the electron beam 20 to executea predetermined scan of information. At this time interrogate pulsegenerator 76 is coupled to control grid 16 by way of switch 72 andinformation is read out, the signal being derived from signal plate 50and amplified in amplifier 54. The resultant pulse information includinga plurality of pulses 60 is transmitted over telephone line 70 tostorage tube 86. Pulses 60 are of sufficient duration and the constantsof storage tube 86 are such that the information is written upon thetarget of storage tube 86.

The telephone line transmission system herein employed may be ofrelatively narrow bandpass characteristics, and the scanning rateemployed can be quite slow to accommodate such characteristics. Forexample, the scanning rate may be reduced to almost zero. It is notedthe amplitude of pulses 60 are standard and the mixing ofsynchronization signals 66 therewith is therefore simplified.

While I have shown and described a preferred embodiment of my invention,it will be apparent to those skilled in the art that many changes andmodifications may be made without departing from my invention in itsbroader aspects. I, therefore, intend the appended claims to cover allsuch changes and modifications as fall within the true spirit and scopeof my invention.

I claim:
 1. Charge image storage apparatus comprising:a storage targethaving capacitance for storing a charge pattern, first means forbombarding such storage target with a first beam of electrons capable ofcausing secondary emission from selected areas from said storage targetupon which said beam is directed for changing the voltage at suchselected areas to store information, second means for bombarding thestorage target with a lower velocity flood beam of electrons acting todrive areas of said target towards one of two stable potentialsdepending upon information stored thereon by said first means for thepurpose of holding such information on the target, means for modulatingsaid first beam to read out stored potential information located at aselected area of said target at which the first beam is then directedincluding means for .Iadd.independently .Iaddend.pulsing said firstbeam, wherein each pulsation thereof has a duration less than necessaryto change such selected area from one said stable potential to another,the said flood beam driving said area back to its stable potentialduring the time between pulsations in order to retain such stablepotential of such area, .[.and.]..Iadd. means for systematicallyscanning said first beam of electrons, during read-out, over areas ofthe target where information is stored, and .Iaddend. means coupled tosaid target for detecting a read-out signal.
 2. A charge image storageapparatus comprising:a storage target for storing a charge pattern,means for bombarding said storage target with an electron beam having avelocity capable of causing secondary emission from selected areas ofsaid storage target upon which said beam is directed to change thevoltage at such selected areas for storing information, and forbombarding sasid storage target with a beam of electrons for reading outstored information at a selected area of said target at which such beamis then directed, including means for .Iadd.independently.Iaddend.pulsing said beam during read-out, .[.and.]..Iadd. means forsystematically scanning said beam of electrons, during read-out, overareas of the target where information is stored, and .Iaddend. means fordirecting a flood beam of electrons at such target for driving areas ofsuch target towards one of two stable potentials by continued secondaryemission from areas where information has been previously stored bysecondary emission and by driving such target negative in areas whereininformation has not been previously stored by secondary emission,wherein such pulsation during read-out has a duration less thannecessary to change a selected area from one stable potential toanother, the flood beam of electrons driving such area back towards itsstable potential in order to retain such stable potential of such area..[.3. The apparatus according to claim 2 wherein such pulsation isperiodic and the period between successive pulsations is sufficient forsaid flood beam to retain the area being read out at its storedpotential by driving such area entirely back to its normally storedpotential whereby the sequence of pulsations does not drive such areafrom one stable potential to another..].
 4. The apparatus according toclaim 2 wherein said storage target comprises a photosensitive storagedielectric such that information may be stored thereon by means of alight image. .[.5. The apparatus according to claim 2 wherein areas ofsaid target wherein information is stored are systematically scannedduring read-out with a beam of electrons..]. .[.6. The apparatusaccording to claim 5 including detection means coupled to said targetfor providing a read-out signal corresponding to said pulsations andindicative of information stored on the systematically scanned areas ofsaid target, andincluding a second charge image storage apparatuscomprising a storage tube disposed at a location remote from said firstmentioned charge image storage apparatus, and communication meansbetween the first mentioned charge image storage apparatus and thecharge image storage apparatus at the remote location including meansfor synchronizing the systematic scan at both such locations, suchcommunication means coupling the output from the detection means of thefirst mentioned charge image storage apparatus to the second chargeimage storage apparatus for storing apparatus for storage at the latterlocation..]. .[.7. The apparatus according to claim 2 wherein the timeduration, t, for a pulsation is less than the product of the voltage atthe first crossover point in the secondary emission characteristic ofthe target and the capacitance of a target area read out, said productbeing divided by the sum of the reading beam current multiplied by (1minus the secondary emission ratio for the reading beam) and the floodbeam current multiplied by (1 minus the secondary emission ratio for theflood beam)..].
 8. The apparatus according to claim .[.7.]. .Iadd.30.Iaddend.wherein said pulsation is repetitive and the period betweenpulsations is greater than the product of the voltage at the firstcrossover point in the secondary emission characteristic for the targetand the capacitance of the target area being read out, said productbeing divided by the floor beam current multiplied by (1 minus thesecondary emission ratio for the flood beam).
 9. The apparatus accordingto claim .[.7.]. .Iadd.30 .Iaddend.wherein said pulsation is repetitiveand the period between pulsations is greater than the product of avoltage, V, and the capacitance of the target area being read out, saidproduct being divided by the flood beam current multiplied by (1 minusthe secondary emission ratio for the flood beam), wherein the voltage,V, is the voltage actually reached by the target area being read outduring the time, t. Charge image storage apparatus comprising:a storagetarget including a photosensitive storage dielectric of phosphormaterial, a first electron gun for bombarding said storage dielectricand writing information thereon during a writing mode with a first beamof electrons capable of causing secondary emission from selected areasof said storage target upon which said beam is directed to change thevoltage at such selected areas toward a more positive value for therebystoring information, flood gun means for bombarding the storagedielectric with a flood beam of low velocity electrons to causecontinued secondary emission from said storage dielectric in the regionwhere said first beam has been directed in its writing mode whilemaintaining other regions negative for providing two stable dielectricpotentials on said target, said first electron gun including a controlgrid having read-out means coupled thereto for .Iadd.independently.Iaddend.modulating said first beam in a reading mode such that saidfirst beam emits regular pulsations of electrons, each pulsation havinga duration less than necessary to change a selected area from one stablepotential to another during read-out of stored information, said floodgun producing a flood beam driving the area read back towards its stablepotential, and signal plate means coupled to said target for providing aread-out signal, said charge image storage apparatus being provided withdeflection means and scanning signal generator means coupled to saiddeflection means to cause said first electron beam from said firstelectron gun to systematically and periodically scan during a readingmode the areas of said target where information is stored.
 11. Theapparatus according to claim 10 further including amplifier meanscoupled to said signal plate means for receiving the read-outsignal,pulse stretching means coupled to receive the output of saidamplifier means for increasing the duration of pulsations received bysaid amplifier means indicative of information stored on said target,mixing means for mixing said stretched pulsations with synchronizationinformation from said scanning means, a storage tube at a locationremote from said charge image storage apparatus, transmission meansbetween said mixing means and said storage tube at said remote location,scan synchronization means coupled to said transmission means at saidremote location causing a systematically and periodic scan in saidstorage tube synchronized with the systematic and periodic scan in saidcharge image storage apparatus, and means applying the pulsations fromsaid transmission means to said storage tube as a writing input thereofso that information corresponding to the bistable images in said chargeimage storage apparatus is written in said storage tube.
 12. Theapparatus according to claim 11 further including a bandpass filter incascade with said transmission means.
 13. A method of readinginformation from charge image storage apparatus including a dielectricstorage target, a high velocity electron beam means directing a highvelocity beam at said storage target, a low velocity flood gun meansdirected at said storage target for retaining information thereon, andmeans coupled to said target for providing a read-out signal, saidmethod comprising:directing a high velocity beam from said firstelectron gun at a particular area of said storage target for readinginformation from the same, .Iadd.independently .Iaddend.pulsing saidelectron beam from said first electron gun with a pulse having aduration less than necessary to change an area being read out from onestable potential to another such stable potential, repeating such pulsewith a frequency such that the period between pulses is sufficient forsaid flood beam to retain such area at a stored potential value bydriving such area back towards its stored potential, .[.and.]..Iadd.scanning said electron beam in a regular pattern across the areas ofsaid target storing information at the same time such beam is pulsed,and .Iaddend. deriving from said means coupled to said target, read-outpulsations corresponding to pulses applied to said first electron gunand indicative of the polarity of stored information. .[.14. The methodaccording to claim 13 further including scanning said electron beam in aregular pattern across the areas of said target storing information atthe same time such beam is pulsed..]. .[.15. The method according toclaim 13 wherein the duration of a pulse is less than the product of thevoltage at the first crossover point in the secondary emissioncharacteristic of said target and the capacitance of the area of saidtarget being read out, said product being divided by the sum of thefirst beam current multiplied by: (1 minus the secondary emission ratiofor the beam from the first electron gun), and the flood beam currentmultiplied by (1 minus the secondary emission ratio for the flood beam)and wherein the period between pulses is greater than the product of thevoltage at the first crossover in the secondary emission characteristicof the target and the capacitance at the area being read out, saidproduct being divided by the flood beam current multiplied by (1 minussecondary emission ratio for the flood beam)..]. The method of claim.[.15.]. .Iadd.31 .Iaddend.wherein said electron beam is scanned in aregular pattern and wherein the duration of pulsation of said beam islengthened and the period between pulses is shortened as the beamscanning rate is increased. .Iadd.
 17. Image storage apparatuscomprising:a tube adapted to store an image, said tube including astorage target and means for directing an electron beam toward saidtarget for reading out stored information at selected areas of saidtarget and including means for independently pulsing said beam duringread-out, wherein the duration of a pulsation is less than would causestored image information to be removed and new information written,means for systematically scanning said electron beam, during read-out,over areas of the target where information is stored, means coupled tosaid target for providing a read-out signal corresponding to saidpulsations and indicative of information stored on the systematicallyscanned areas of said target, utilization apparatus for providing arepresentation of information stored on the systematically scanned areasof said target, and communication means between said means for providinga read-out signal and said utilization apparatus for coupling the outputfrom the means for providing a read-out signal to said utilizationapparatus. .Iaddend..Iadd.
 18. The apparatus according to claim 17further including means for synchronizing the systematic scan of theareas of said target where information is stored and the operation ofsaid utilization apparatus for providing a representation of theinformation stored. .Iaddend..Iadd.
 19. The apparatus according to claim18 wherein said means for directing an electron beam at said targetincludes an electron gun and said means for pulsing said beam includes acontrol grid for said gun and means coupled to said control gridproviding a control input for modulating said beam with regularpulsations. .Iaddend. .Iadd.
 20. Image storage apparatus comprising: atube having storage target, and an electron gun for bombarding saidtarget with a narrow beam of electrons, said electron gun including acontrol grid provided with an input for independently modulating saidbeam such that said beam emits regular pulsations of electrons, eachpulsation having a duration less than would cause removal of informationstored on said storage target, means coupled to said target forproviding a read-out signal, scanning means for causing said electronbeam to systematically and periodically scan areas of the target whereinformation is stored and is to be read out, utilization means forproviding a representative of information stored on systematicallyscanned areas of said target, transmission means between said signalplate means and said utilization means, and scan synchronization meanscoupled to said scanning means and to said utilization means forsynchronizing operation of said utilization means with a systematic andperiodic scan of the electron beam. .Iaddend..Iadd.
 21. The apparatusaccording to claim 20 wherein said utilization means comprises a cathoderay tube having deflection means and adapted to have the electron beamthereof deflected under the control of said synchronization meanssubstantially in step with the first mentioned beam of electrons fromsaid electron gun. .Iaddend. .Iadd.
 22. Image storage apparatuscomprising: a storage target, means for bombarding said storage targetwith an electron beam for reading out stored information at selectedareas of said target at which such beam is then directed including meansfor independently pulsing said electron beam, wherein each pulsationthereof during normal read-out has a duration less than would remove thestored information, and means coupled to said target for detecting aread-out signal in response to pulsation of said beam, wherein saidcharge image storage apparatus is provided with deflection means forcausing systematic scan of said electron beam over areas of the targetwhere information is stored and which it is desired to read out..Iaddend. .Iadd.
 23. The apparatus according to claim 22 including meansfor causing a beam of electrons to bombard said target for storinginformation on said target. .Iaddend..Iadd.
 24. The apparatus accordingto claim 22 wherein said storage target comprises photosensitivematerial such that information may be stored thereon by means of a lightimage. .Iaddend..Iadd.
 25. The apparatus according to claim 22 whereinsaid means for bombarding said storage target with an electron beamcomprises an electron gun and wherein said means for pulsing saidelectron beam comprises a control grid for said electron gun havingmeans coupled thereto for modulating said beam with regular voltagepulsations. .Iaddend. .Iadd.
 26. The method of reading information froman image storage apparatus including a target, means for directing abeam of electrons at said target, and means coupled to said target forproviding a read-out signal, said method comprising: directing a beam ofelectrons from the first mentioned means at a particular area of thetarget from which it is desired to read information, independentlypulsing said electron beam with a pulse duration less than would removethe stored information from the target, scanning the electron beam in aregular pattern across areas of the target storing information at thesame time the beam is pulsed, and deriving, from said means coupled tosaid target, read-out pulsations corresponding to pulses applied to thebeam. .Iaddend. .Iadd.
 27. The method according to claim 26 furtherincluding reproducing the stored information at a second location inresponse to the read-out pulsations derived from said means coupled tosaid target. .Iaddend. .Iadd.
 28. A charge image storage apparatuscomprising: a storage target for storing a charge pattern, means forbombarding said storage target with an electron beam having a velocitycapable of causing secondary emission from selected areas of saidstorage target upon which said beam is directed to change the voltage atsuch selected areas for storing information, and for bombarding saidstorage target with a beam of electrons for reading out storedinformation at a selected area of said target at which such beam is thendirected, including means for pulsing said beam during read-out, meansfor systematically scanning said beam of electrons, during read-out,over areas of the target where information is stored, and means fordirecting a flood beam of electrons at such target for driving areas ofsuch target towards one of two stable potentials by continued secondaryemission from areas where information has been previously stored bysecondary emission and by driving such target negative in areas whereinformation has not been previously stored by secondary emission,wherein such pulsation during read-out has a duration less thannecessary to change a selected area from one stable potential toanother, the flood beam of electrons driving such area back towards itsstable potential in order to retain such stable potential of such area,and including means for causing said pulsation to be periodic and theperiod between successive pulses to be sufficient for said flood beam toretain the area being read out at its stored potential by driving sucharea entirely back to its normally stored potential whereby the sequenceof pulsations does not drive such area from one stable potential toanother. .Iaddend..Iadd.
 9. A charge image storage apparatuscomprising:a storage target for storing a charge pattern, means forbombarding said storage target with an electron beam having a velocitycapable of causing secondary emission from selected areas of saidstorage target upon which said beam is directed to change the voltage atsuch selected areas for storing information, and for bombarding saidstorage target with a beam of electrons for reading out storedinformation at a selected area of said target at which such beam is thendirected, including means for pulsing said beam during read-out, meansfor systematically scanning said beam of electrons, during read-out,over areas of the target where information is stored, means fordirecting a flood beam of electrons at such target for driving areas ofsuch target towards one of two stable potentials by continued secondaryemission from areas where information has been previously stored bysecondary emission and by driving such target negative in areas whereinformation has not been previously stored by secondary emission,wherein such pulsation during read-out has a duration less thannecessary to change a selected area from one stable potential toanother, the flood beam of electrons driving such area back towards itsstable potential in order to retain such stable potential of such area,detection means coupled to said target for providing a read-out signalcorresponding to said pulsations and indicative of information stored onthe systematically scanned areas of said target, and including a secondcharge image storage apparatus comprising a storage tube disposed at alocation remote from said first mentioned charge image storageapparatus, and communication means between the first mentioned chargeimage storage apparatus and the charge image storage apparatus at theremote location including means for synchronizing the systematic scan atboth such locations, such communication means coupling the output fromthe detection means of the first mentioned charge image storageapparatus to the second charge image storage apparatus for storage atthe latter location. .Iaddend..Iadd.
 30. A charge image storageapparatus comprising: a storage target for storing a charge pattern,means for bombarding said storage target with an electron beam having avelocity capable of causing secondary emission from selected areas ofsaid storage target upon which said beam is directed to change thevoltage at such selected areas for storing information, and forbombarding said storage target with a beam of electrons for reading outstored information at a selected area of said target at which such beamis then directed, including means for pulsing said beam during read-out,means for systematically scanning said beam of electrons, duringread-out, over areas of the target where information is stored, andmeans for directing a flood beam of electrons at such target for drivingareas of such target towards one of two stable potentials by continuedsecondary emission from areas where information has been previouslystored by secondary emission and by driving such target negative inareas where information has not been previously stored by secondaryemission, wherein such pulsation during read-out has a duration lessthan necessary to change a selected area from one stable potential toanother, the flood beam of electrons driving such area back towards itsstable potential in order to retain such stable potential of such area,wherein the time duration, t, for a pulsation is less than the productof the voltage at the first crossover point in the secondary emissioncharacteristic of the target and the capacitance of a target area readout, said product being divided by the sum of the reading beam currentmultiplied by (1 minus the secondary emission ratio for the readingbeam), and the flood beam current multiplied by (1 minus the secondaryemission ratio for the flood beam). .Iaddend. .Iadd.
 31. A method ofreading information from charge image storage apparatus including adielectric storage target, a high velocity electron beam means directinga high velocity beam at said storage target, a low velocity flood gunmeans directed at said storage target for retaining information thereon,and means coupled to said target for providing a read-out signal, saidmethod comprising: directing a high velocity beam from said firstelectron gun at a particular area of said storage target for readinginformation from the same, pulsing said electron beam from said firstelectron gun with a pulse having a duration less than necessary tochange an area being read out from one stable potential to another suchstable potential, repeating such pulse with a frequency such that theperiod between pulses is sufficient for said flood beam to retain sucharea at a stored potential value by driving such area back towards itsstored potential, scanning said electron beam in a regular patternacross the areas of said target storing information at the same timesuch beam is pulsed, and deriving from said means coupled to saidtarget, read-out pulsations corresponding to pulses applied to saidfirst electron gun and indicative of the polarity of stored information,wherein the duration of a pulse is less than the product of the voltageat the first crossover point in the secondary emission characteristic ofsaid target and the capacitance of the area of said target being readout, said product being divided by the sum of the first beam currentmultiplied by (1 minus the secondary emission ratio for the beam fromthe first electron gun), and the flood beam current multiplied by (1minus the secondary emission ratio for the flood beam) and wherein theperiod between pulses is greater than the product of the voltage at thefirst crossover in the secondary emission characteristic of the targetand the capacitance at the area being read out, said product beingdivided by the flood beam current multiplied by (1 minus the secondaryemission ratio for the flood beam). .Iaddend. .Iadd.
 32. A charge imagestorage apparatus comprising: a storage target for storing a chargepattern, means for bombarding said storage target with an electron beamhaving a velocity capable of causing secondary emission from selectedareas of said storage target upon which said beam is directed to changethe voltage at such selected areas for storing information, and forbombarding said storage target with a beam of electrons for reading outstored information at a selected area of said target at which such beamis then directed, including means for pulsing said beam during read-out,means for systematically scanning said beam of electrons, duringread-out, over areas of the target where information is stored, meansfor directing a flood beam of electrons at such target for driving areasof such target towards one of two stable potentials by continuedsecondary emission from areas where information has been previouslystored by secondary emission and by driving such target negative inareas where information has not been previously stored by secondaryemission, wherein such pulsation during read-out has a duration lessthan necessary to change a selected area from one stable potential toanother, the flood beam of electrons driving such area back towards itsstable potential in order to retain such stable potential of such area,detection means coupled to the charge image storage apparatus forproviding a read-out signal corresponding to said pulsations andindicative of information stored on the systematically scanned areas ofsaid target, a cathode ray tube disposed at a location remote from saidfirst mentioned charge image storage apparatus, and communication meansbetween the charge image storage apparatus and said cathode ray tube atsaid remote location including means for synchronizing the systematicscan at both such locations, said communication means coupling theoutput from the detection means of the charge image storage apparatus tothe cathode ray tube at the latter location. .Iaddend..Iadd.
 33. Theapparatus according to claim 32 further including pulse stretching meansfor stretching the output of said detecting means. .Iaddend.